Flash modules in cellular telephones that incorporate a camera are now commonplace. Such flash modules must be thin since a desirable feature of cell phones is their small size. Many types of cell phone cameras also take digital movies, which may require the illuminator to be constantly on while the video is being recorded. The illuminator for flashes and video illumination will be referred to as a flash for simplicity. As used herein, the term camera refers to a still picture camera and/or a video recorder.
To improve reliability, reduce cost, and reduce size, it is known to replace the conventional, non-solid state flash bulbs with a high power light emitting diode (LED) that emits white light.
Bulbs (e.g., xenon) are relatively large and use a curved mirror to direct light toward the subject. This results in a relatively deep bulb module that is objectionable when extreme thinness is desirable. Such a module is also relatively expensive and requires a special high voltage generator, which is large. The arc produced in the bulb is an intense point of light that concentrates all the light output power in substantially a point source.
LEDs used as flashes are also typically mounted in a mirrored bowl. The LEDs output light in a Lambertian pattern, and the curved mirror redirects light toward the subject. Even though LED dice are very thin, the dice are typically mounted on a submount and packaged. The package contains a reflector and lens, and the overall package typically has a thickness on the order of 5 mm. The package is mounted on a printed circuit board, which adds more thickness to the module. The LED flash is substantially a point source, since the LED die is about 1×1 mm.
The intense point of light emitted by the bulb or LED is objectionable to the subject being photographed. If the light were diffused by a diffusing lens separated from the point light source, the thickness of the overall light module would greatly increase, and the extreme diffusing needed for a point source would inherently result in much of the point source light output being absorbed by the diffuser or reflected back toward the light source.
What is needed is an extremely thin illuminator for cameras, especially cell phone cameras, where the illuminator light output is spread over a relatively large area.